Ice-two
(Ice II)

Ice-two (ice II)
may be synthesized from hexagonal ice at 198 K and 300 MPa or by decompressing ice-five (ice V) at 238 K but is not easily formed by cooling ice-three (ice III)
(see Phase Diagram). It may form
a major proportion of icy moons such as Jupiter's Gannymede
[839].

Its unit cell, which forms rhombohedral
crystals, (Space group ) is shown opposite. In the crystal,
all water molecules are hydrogen bonded to four others,
two as donor and two as acceptor. Ice-two may exist
metastably below ~100 K between ambient pressure and
~5 GPa. At ambient pressure it irreversibly transforms
into ice Ic above 160 K. As the H-O-H angle does not vary much
from that of the isolated molecule, the hydrogen bonds
are not straight (although shown so in the figures).

Half the open hexagonal channels of ice Ih
have collapsed in ice II.
The relationship of the ice II structure to ice Ih
can be visualized by detaching the columns of hexameric
ice Ih
rings, moving them relatively up or down at right angles
to their plane, rotating them about 30° around this
axis and re-linking the hydrogen bonds in a more compact
way to give a density of 1.16 g cm-3. The hydrogen
bonding is ordered and fixed in ice-two, as can be seen
in the linkages round the hexamers. There is no corresponding
disordered phase, in contrast to the other ordered ices VIII, IX, XI and XV. The lack of a disordered phase has been correlated with the high energy difference between the most and the second most stable ice configurations [1655]. Some of ice-two's hydrogen bonds are
bent and, consequentially, much weaker than the hydrogen
bonds in hexagonal ice.